1. Field of the Invention
The present invention relates to an image forming device for forming image data on a photosensitive drum in an electronic printing apparatus for example, with the use of a line head comprised of edge-emission type electroluminescent elements.
2. Description of the Related Art
Generally, an edge-emission type EL (electro-luminescent) element 1 has a multi-layer structure as shown in FIG. 7. The multi-layer structure comprises a thin film-like active layer 2 formed of an active element-containing zinc sulfide, dielectric layers 3, 4 surrounding the active layer 2 and flat plate electrodes 5 and 6 formed on the upper and lower surfaces of the dielectric layers 3 and 4, respectively. The edge-emission type EL element 1 is used for a line head for forming an exposure image on a photosensitive body of an electric field emission type printer.
FIG. 8 shows a relation between the timing of an electric pulse P applied to the edge emission type EL element 1 and a quantity of light emitted from the EL element 1 at that time. At a point of time, A, a potential difference exceeding a threshold voltage vth is applied across the plate electrodes 5 and 6 in the EL element 1 to emit light from the EL element 1. At this time, the quantity of light emitted rises abruptly. Upon arrival at a predetermined point of time, B, the light quantity is gradually declined.
As shown in FIG. 9, a line head 7 is formed of one array of the edge-emission type EL elements 1 provided on a substrate 8. A rod lens array, not shown, is arranged in an opposed relation to the forward ends of the EL elements in the line head 7.
The EL elements 1 in the line head 7 are arranged, as one array, in parallel (major scanning direction) to the axial direction of, for example, a cylindrical photosensitive drum, not shown. While the photosensitive drum is rotated by one line corresponding to one pixel width, the associated EL elements 1 emit light a plurality of times at the same cycle.
By so doing, the respective pixel is formed on the photosensitive body through exposure and one line is recorded, by the line head 7, on the photosensitive drum through exposure.
In the conventional image forming device, in order to form one pixel on the photosensitive drum by the EL elements, the EL elements controllably emit light a plurality of times at the same cycle as shown, for example, in JPN PAT APPN KOKAI PUBLICATION 62-106479.
As shown in FIG. 10, for example, upon light emission of the EL element by the application of one pulse it is not possible to exceed a quantity of light, C, adequate to illuminate the photosensitive drum. Further, even if the pulses are applied to the EL elements continuously three times at a cycle T1, it is not possible to exceed the adequate quantity of light, C, and the quantity of light, C, adequate to illuminate the photosensitive drum can be obtained only through the emission of light by the application of the fourth and subsequent pulses.
That is, since the photosensitive drum is not adequately exposed with emission light by the application of the first few pulses, there is a problem that one pixel is not completely recorded with emission light on the photosensitive drum. As a result, when the image forming apparatus is used for a printer, a thin line, such as a dot line, that is, a pixel line, cannot be clearly printed on a surface.